Career
Koolen made his Hoofdklasse debut at the Twins Sporting Club in 1998. He played for the Dutch junior team in the 1999 World Junior Championship and 2000 World Junior Championship. In the 2004 playoffs, he was named Most Valuable Player. During 2003 and 2004, he spent time in College in the United States.
He was 3 for 17 with 3 walks in 16 games for the 2003 VCU Rams.
In 2004, he hit.210/.300/.290 in 26 games. Returning to the Netherlands full-time in 2005, the 23-year-old hit.253/.345/.326 in 26 games for Neptunus and fielded.889.
Koolen was 4 for 9 in the 2006 Haarlem Baseball Week. After his move to Kinheim for the 2006 Hoofdklasse season, Koolen hit.324/.435/.426 with 27 walks in 42 games.
He was 5th in the league in OBP and tied Eugene Kingsale, Tjerk Smeets and Evert-January "t Hoen for fifth in walks.
He was one for two with a run and Reserve Bank of India as Michael Duursma"s backup at 2B in the 2006 Intercontinental Cup. In 2007, Koolen hit.386/.525/.534 with 22 walks and 21 runs in 26 games. He lost the batting title by six points to Danny Rombley, was second to Fausto Alvarez in slugging and led the league in OBP. He tied for second in the league in triples (3).
He was 6 for 23 in the 2007 World Portuguese Tournament.
Foreign the 2007 Baseball World Cup, Koolen replaced Yurendell de Caster at second base for the Dutch national team He started ahead of Michael Duursma, who had had a much better career in National Collegiate Athletic Association Division I than Koolen.
Koolen hit.267/.267/.267 in 8 games but the Netherlands still finished 4th, tying their best finish ever. Koolen was selected by coach Robert Eenhoorn in the team that represents the Netherlands at the 2008 Summer Olympics in Beijing.